1. Field of the Invention
The invention relates to apparatus for an improved ISDN terminal adapter, commonly and loosely referred to as an "ISDN modem", and accompanying methods for use therein. Specifically, to simplify and facilitate installation and provide economical use, the inventive terminal adapter provides automatic ISDN switch detection, automatic SPID (service profile identification) configuration, baud rate unblocking and automatic data compression.
2. Description of the Prior Art
In recent years, a number of domestic and foreign telephone companies have begun offering integrated service digital network (ISDN) services to their customers. ISDN provides an integrated voice and data network that offers both increased bandwidth and significant flexibility over traditional analog telephone services. Inasmuch as subscriber charges for ISDN access are decreasing--with the decrease being rather noticeable for some telephone companies, demand for ISDN service and equipment is rising appreciably. Demand is particularly strong and growing for those subscribers who seek cost-effective high speed access to the Internet.
In particular, a basic rate (so-called "2B+D" service) ISDN interface provides higher speed bandwidth than both traditional analog, modem-based dial-up access modalities and comparably priced switched digital services. Each so-called B ("bearer") channel, which carries subscriber voice and/or data, provides 64 Kbits/second of bandwidth; while a D ("data") channel, which carries signaling and control information, provides 16 Kbits/second of bandwidth. For the bandwidth delivered, an ISDN line is significantly less expensive than a private leased line that supplies the same bandwidth across the three channels. Furthermore, ISDN, being a digital end-to-end service, provides digital transmission channels that tend to be more accurate and reliable, from the standpoint of error rates and dropped connections, than are conventional analog connections. In addition, ISDN service provides rapid connect times which, in turn, provide faster support for those LAN (local area network) protocols that require relatively short latency across WAN (wide area network) connections.
Starting a few years ago, various networking and communications equipment manufacturers have been offering relatively inexpensive ISDN terminal adapters, more commonly and rather loosely referred to as "ISDN modems" (though these adapters do not contain a traditional analog modulator-demodulator as occurs in a conventional analog modem), for subscriber end-use. Such a terminal adapter, also generically referred to as "data circuit terminating equipment" (DCE), once connected to an ISDN connection and a serial port on a subscriber's personal computer (PC), permits that subscriber to connect his(her) computer to, e.g., an Internet service provider and communicate at speeds approximately two to four times greater than a conventional analog modem. The computer so connected becomes so-called "data terminal equipment" (DTE). While the availability of these terminal adapters is clearly not the sole cause underlying the growth in ISDN usage, it, when combined with decreasing rates for ISDN service, is certainly a large and growing factor.
Ideally, an ISDN terminal adapter should be as easy for a subscriber to install and use as is a conventional analog modem. However, in practice and for various reasons, installing a conventional ISDN terminal adapter is rather tedious, frustrating and time consuming.
First, user difficulties arise due to differences in so-called "SPID" formats across different ISDN switches. While, for the most part, ISDN service is quite uniform across the entire United States, slight differences do exist as to how this service is implemented among various telephone companies. In particular, a small number of ISDN switch manufacturers currently exist which supply such switches to telephone companies in the United States. These manufacturers currently include Northern Telecom, Ltd., Siemens Corporation and Lucent Technologies, Inc. Northern Telecom and Siemens currently offer one ISDN switch each (carrying product designations "DMS 100" and "EWSD", respectively)--each being a so-called "NI-1" type (referring to "National ISDN 1") switch and will be so referred to hereinafter. The Northern DMS 100 switch is available in two versions, i.e., an NI-1 version and a custom version--those versions differing only in their software. Lucent Technologies currently offers three different ISDN switches (a "5E Custom Multi-point", "5E NI-1" and "5E Custom Point-to-point"), though all implemented on a common 5ESS hardware platform (these switches differing in their generics, i.e., software). At present, there are implementational differences in the United States among ISDN switches currently in service; though no such differences exist in Europe. Consequently, while these switches are generally compatible with each other, idiosyncratic differences do exist among the various switches that complicate installation and use of an ISDN terminal adapter. Specifically, while each switch requires each specific DCE that is to be connected to the switch to be electronically identified by a corresponding SPID (service profile identifier) assigned to it by a local telephone company, the format of the SPID varies from one switch type to another. A SPID established for use with one type of ISDN switch will not likely function with another type of ISDN switch; thus, completely preventing any ISDN calls from being established through the latter switch. Moreover, when an ISDN line is ordered from the local telephone company, a SPID is established for that line and, then, generally, entered manually, as data, into a database on the ISDN switch at the company to provision the service.
Establishing and entering a SPID into a telephone company database is a rather laborious process, and one, for telephone company personnel, that has proven to be a recurring source of errors. Oftentimes, these errors are not detected during data entry. While installing and, specifically, configuring a conventional ISDN terminal adapter for use with an ISDN line, an ISDN subscriber must manually enter a SPID provided by his(her) telephone company for that line into the terminal adapter. During call setup, the adapter transmits its SPID to the switch in order to identify itself to the switch. A significant probability exists that this SPID will be incorrect. Hence, it is the subscriber, during installing his(her) ISDN terminal adapter, who often experiences first-hand the effect of an incorrect SPID. Since the vast majority of subscribers have no knowledge of ISDN control messages, let alone the particular type of ISDN switch to which (s)he is connected and its corresponding SPID format, many subscribers quickly discover that their ISDN terminal adapters will not function, for reasons that are not immediately apparent to or resolvable by them, and hence can become very frustrated and rather irritated. Such subscribers can and often do expend considerable time and effort in telephone conversations with customer service representatives of the local telephone company and/or their terminal adapter manufacturers in an effort to locate and cure the problem--a SPID error. As a result, conventional ISDN terminal adapters have acquired a reputation, at least in the United States and Canada, as being quite time-consuming to install and properly configure. To complicate matters, the same difficulties arise if the subscriber merely moves the ISDN terminal adapter to a different location, i.e., one served by a switch that requires a different SPID format.
Second, while conventional ISDN terminal adapters provide a so-called "autobaud" capability, baud rate "blocking" can occur which causes user difficulties. Many ISDN terminal adapters are externally connected to a personal computer through a serial port provided by the computer. Circuitry used to implement this port, specifically the UART (universal asynchronous receiver transmitter) used therein, supports a wide range of serial data speeds up to approximately 230 Kbaud/second. The speed at which a serial port will communicate is typically defined by a user who selects one of a number of speed settings provided by a serial port driver within an operating system, such as the "Windows 95" or "Windows NT" operating system which is currently available from Microsoft Corporation of Redmond, Wash. (which also owns the trademarks "Windows 95" and "Windows NT"). For proper serial communication, both the serial port on the personal computer and the terminal adapter itself must be configured to communicate at the same speed. Generally, the serial port of the terminal adapter is set to a factory default value which permits a so-called "autobaud" operation to occur on power-up of the adapter. During autobaud operation, the terminal adapter will sample the incoming serial data stream at least six times faster than its data rate to determine the baud rate of the stream. Once the rate is determined, the terminal adapter will set its serial data rate to match that of the stream. Owing to design choices, an ISDN terminal adapter generally does not have a processor that is sufficiently powerful to sample, let alone accurately, an incoming bit stream at six times 230 Kbaud/sec (approximately 1.38 Mbaud/sec). Consequently, conventional ISDN terminal adapters only provide autobaud capability to approximately 115 Kbaud/second. Therefore, in order for a subscriber to use the 230 Kbaud/sec speed, that subscriber must first establish communication between the personal computer and the adapter at a lower speed, i.e., 115 Kbaud/sec or below, and then manually issue a well-known "AT" command from the computer to the adapter to manually set and lock the speed of the adapter at 230 Kbaud/second. The terminal adapter retains this setting within its non-volatile memory to appropriately set its speed during subsequent power-on initialization. Thereafter, the subscriber changes the speed setting in his(her) serial port communication driver to 230 Kbaud/second, hence matching that of the adapter. Ser. communication can then occur between the personal computer and the terminal adapter at a maximum rate of 230 Kbaud/second. Unfortunately, a serious problem occurs if the subscriber decides to move the terminal adapter to a different computer. Since the adapter is locked to communicate at only 230 Kbaud/second and will not "autobaud", then, if the serial port driver on that computer is set to any speed other then 230 Kbaud/second or simply cannot support a speed of 230 Kbaud/sec--regardless of its setting, the terminal adapter and the computer will simply not communicate with each other; i.e., communication is "blocked" by the baud rate difference. A user of a conventional terminal adapter generally has no idea that baud rate "blocking" is occurring and hence becomes quite frustrated. Generally speaking, baud rate "blocking" can occur at any other baud rate, i.e., less than 230 Kbaud/sec, whenever the baud rate of the terminal adapter is set and locked to one speed while the serial port of the PC to which the adapter is connected is set to a different baud rate.
Therefore, a need exists in the art for an ISDN terminal adapter that significantly simplifies and expedites its installation and configuration. Preferably, such a adapter should automatically detect the type of ISDN switch to which the adapter is connected and adjust its operation to assure compatibility with that switch. In addition, such an adapter should detect when a condition of baud rate blocking likely occurs, regardless of the baud rate to which the terminal adapter is set, and automatically change its operation to terminate the condition. Advantageously, such an adapter will not only substantially eliminate a burden heretofore shouldered by an ISDN subscriber but also, by doing so, might increase the overall demand for ISDN service.